I was having a hard time figuring out what blog my upgrade process had stopped on, since WP wasn’t telling me. Everything I found on the internet said it would go through the blogs in order, but that clearly wasn’t the case.
With a bit of sleuthing in my database, I realized it was going through the blogs in reverse order – the most recently created were being updated first.
When I visited the blogs, I got a “database connection error” message. When I tried to visit the blogs’ wp-admin pages, I was told that I needed to repair the database tables. When I told WP to do that, it turned up three blogs (one at a time) that were missing all of their database tables. (I have no idea why.) Once I deleted the references to those blogs in the wp_blogs table, my upgrade proceeded smoothly.
I hope that information helps someone else out there! I had a beast of a time finding any information on this problem.
